Sunday, September 18, 2011

Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimetres (24 in) in length on the largest specimens. They are also known as serpent stars.

Ophiuroidea contains two large clades, Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group.

There are some 1,500 species of brittle stars living today, and they are largely found in deep waters more than 500 metres (1,650 feet) down.

Reproduction
The sexes are separate in most species, though a few are hermaphroditic or protandric. The gonads are located in the disc, and open into pouches in between the arms, called genital bursae. Fertilisation is external in most species, with the gametes being shed into the surrounding water through the bursal sacs.[1] An exception is the Ophiocanopidae, in which the gonads do not open into bursae and are instead paired in a chain along the basal arm joints.

Many species brood developing larvae in the bursae, effectively giving birth to live young. A few, such as Amphipholus squamata are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. However, there are some species that do not brood their young, instead having a free-swimming larval stage. Referred to as an ophiopluteus, these larvae have four pairs of rigid arms lined with cilia. They develop directly into an adult, without the attachment stage found in most starfish larvae.[1] The number of species exhibiting ophiopluteus larvae are fewer than those that directly develop.

In a few species the female carries a dwarf male, clinging to it with the mouth